1. Technical Field
This invention relates generally to Integrated Services Digital Network (ISDN) interface devices. More particularly, the invention provides a technique for simplifying the bundling of ISDN lines within an office building to provide increased data bandwidths for video conferencing and other applications.
2. Related Information
An important application of ISDN (Integrated Services Digital Network) is the transmission of video conferencing data and other computer-to-computer data. A single ISDN line (a "Basic Rate Interface" or "BRI") typically includes two full-duplex channels referred to as "2B+D". The two B channels are for digital voice or data, and each supports a data rate 64 kilobits/second. The D channel is used to send and receive control messages from the telephone company's Central Office (CO) and supports a data rate of 16 kilobits/second. One ISDN line can actually carry two telephone calls simultaneously on each of the two B channels. Alternatively, ISDN lines can be used to transmit video data such as for a video conferencing application.
Videoconferencing users have found that a data rate of 64 kilobits/second is not sufficient for quality video transmissions. One prior art technique for increasing the data rate (and hence the video quality) has been to "bundle" two ISDN lines together for a single video conference call. Thus, for example, conventional videoconferencing equipment can use both B channels of an ISDN line simultaneously, effectively providing a video conferencing bandwidth of 128 kilobits per second.
However, human factors studies show that current discrete cosine transform based compression techniques using a bandwidth of 128 kilobits per second is still not enough to transmit a high quality video image. Although the video quality is improved over 64 kilobits/second, most business users still complain that the video images are too "jerky". Only 20% of typical users feel that 128 kilobit video is acceptable or useful. By increasing the data rate, the video can be improved further. In fact, the same studies have shown that at a bandwidth of 384 kilobits/second, 80% of potential users feel that the video is useful.
In order to obtain higher data rates with ISDN, it is necessary to combine more B channels. For a data rate of 384 kilobits/second, six B channels are needed, which requires three BRI (i.e., three ISDN lines).
A BRI is conventionally delivered by the telephone company to an office building or other user premises over a single unshielded twisted pair (UTP) of wires. All of the data, full-duplex 2B+D, is delivered over the single UTP using a sophisticated adaptive echo-cancellation system. This single UTP line is called a "U-channel".
Once the U-channel enters the user's premises, it is usually converted into an "S/T-Channel", which uses two UTP (i.e., two pairs of wires), one pair for transmitted data and the other pair for received data. Conversion from the U-channel to S/T-channel is conventionally performed with an electronic device called an "NT1" or Network Termination 1 device. This device normally has a separate power supply, one UTP input from the telephone company central office, and two UTP outputs which connect to the terminal equipment.
FIG. 1 shows a typical prior art installation of a single ISDN line or BRI. A customer office facility 101 is coupled to a telephone company central office 102 through a U-channel which, as explained above, comprises a single pair of unshielded twisted pair wires. In this configuration, the telephone company installs standard telephone wall jacks and four-pair cables to deliver the ISDN line to the user's terminal equipment 103, such as a videoconferencing terminal. Such videoconferencing terminals are well known and may include a computer, camera, and telephony interface devices to permit video and audio signals to be bidirectionally transmitted between users.
Conventional videoconferencing terminals may also include electronic equipment which combines multiple ISDN lines and B channels together to provide a 384 kilobit/second or faster data channel for the terminal. This combining system is called an "Inverse Multiplexer" or "IMUX" since it starts by splitting the wider bandwidth signals into 64 kilobit streams for transmission over the ISDN B channels.
FIG. 2 shows a conventional multi-line installation which permits multiple ISDN lines to be combined in order to increase the data rate to terminal equipment such as a videoconferencing terminal 203. However, as shown in FIG. 2, the telephone company must install new four-pair cables from the point of entry in the basement (one for each pair of S/T channels), NT1 devices 205, 206 and 207, and new jacks J1, J2 and J3 in the room with the terminal equipment. Thus, one new NT1 device, a new cable, and a new wall jack is needed for each additional ISDN line. The videoconferencing equipment uses six 64 kilobit per second S/T channels, combined to achieve 384 kilobits per second, to transmit video conferencing data.
The added equipment required to combine S/T channels results in extra expense. Moreover, the time required by the telephone company to install the equipment and additional telephone jacks results in additional labor costs. Consequently, a more efficient method of combining ISDN lines to achieve higher bandwidths is needed.